The present invention relates to the field of electrical engineering in general and more particularly to conversion technique and to methods of current commutation by bi-directional switches of matrix converters, for instance to three phase-to three phase matrix converters with a space vector modulation.
The invention can be used in induction motor drives, special power supplies, etc. More particularly this invention can be used in power supplies on a base of AC-AC converters with control and stabilization of output frequency.
There are well-known methods of current commutation by bi-directional switches which consist of two separately controlled halves having the opposite direction conductivity, that are based on information about polarity of load current (IL) of matrix converters (MC) [P. Wheeler, D. Grant, xe2x80x9cOptimized input filter design and low-loss switching techniques for a practical matrix converterxe2x80x9d, in IEE Proc.-Electr. Power Applicat., vol.144, no.1, pp.53-60, January 1997; L. Empringham., P. W. Wheeler, J. C. Clare, xe2x80x9cMatrix converter bi-directional switch commutation using intelligent gate drivesxe2x80x9d, IEE Power Electronics and Variable Speed Drives Conference, No.456, London, September, 1998], or on information about relation of mains voltages [M. Ziegler, W. Hofmann, xe2x80x9cSemi natural two steps commutation strategy for matrix converterxe2x80x9d, in Rec. 29th Annual IEEE meeting, PESC""98, 1998, vol.1, pp. 727-731; H. Youw, B. H. Kwon, xe2x80x9cSwitching technique for current controlled AC-to-AC convertersxe2x80x9d, IEEE Trans. Ind. Electron., vol. 46, no. 2, pp. 309-318, April 1999].
In case of tracking the IL polarity, halves of switches are being controlled separately for every current polarity. It allows to provide a correct commutation (without short circuit currents and overvoltages under breaking current in the load inductance), if the polarity of IL is single-valued. If current IL≈0 or changes its polarity repeatedly during the period of the output voltage, a method proposed in [P. W. Wheeler, D. Grant, xe2x80x9cOptimized input filter design and low-loss switching techniques for a practical matrix converterxe2x80x9d, in IEE Proc.-Electr. Power Applicat., vol.144, no.1, pp 53-60, January 1997], does not allow to get any exact determination of step-by-step commutations order of switches halves under every changing of current IL polarity. According to [L. Empringham, P. W. Wheeler, J. C. Clare, xe2x80x9cMatrix converter bi-directional switch commutation using intelligent gate drivesxe2x80x9d, IEE Power Electronics and Variable Speed Drives Conference, No.456, London, September, 1998], the polarity of IL is being defined on the difference of voltage across switches halves at every moment of time. It allows to define enough exactly the moment when current falls to zero. But this method requires a considerable increasing of intellectual potential of power switches drivers and the further complication of logical part of control system due to multiple cross connections between drivers of all the switches of every output MC phase. Besides, after the fixing of zero current it appears an uncertainty in subsequent choice of the necessary order of switching the switches halves, and this also complicates a realization of the commutation method.
Under the real conditions, when the output current curve is a high frequency ripple and crosses zero line repeatedly, a method realization of how to transfer current from one switch to another with using mentioned method will be complicated.
Under using methods being based on information of the relation of mains voltages, the voltages period is being divided into intervals with said relation being constant, and it is turned constantly on those halves of all switches which are connected to a load phase and which do not produce paths for short circuit currents during current interval of the mains voltage [M. Ziegler, W. Hofmann, xe2x80x9cSemi natural two steps commutation strategy for matrix converterxe2x80x9d, in Rec. 29th Annual IEEE meeting, PESC""98, 1998, vol.1, pp. 727-731 ], or halves of only two switches being connected to input phases with maximal and minimal voltages within the interval given [H. Youw, B. H. Kwon, xe2x80x9cSwitching technique for current controlled AC-to-AC convertersxe2x80x9d, IEEE Trans. Ind. Electron., vol. 46, no. 2, pp. 309-318, April 1999].
Such approach allows to provide for a MC phase constant being of conditional freewheeling diodes on the analogy with voltage source inverters and gives a possibility to make switches commutation with dead time, because there always is a way for current of both polarity. But in the moments of approximate equality of phase voltages, especially if they are distorted (for instance, as a result of flowing the modulated current being supplied from the mains through filter inductance) and repeatedly are being compared under intervals changing, it appears an uncertainty when fixing boundaries of mains voltage intervals.
The uncertainty in the choice of switches halves which are to be constantly turned on during the interval results in short circuit currents through the switches in case of overlap time, or in current breaking under commutations, if the half of the switch which is necessary for current conducting during dead time appears to be turned off. In the case of significant distortions of input voltages methods mentioned above can not be used.
The prior art of the proposed invention is a method of commutation of current by bi-directional switches of matrix converters with at least three input phases, the switches consisting of two separately controlled halves with the opposite directions conductivity, which implies that under transferring current from the switch being turned on in both directions and connected to one input phase to the switch being connected to another input phase, it is being turned on the half of the switch that does not make a way for flowing short circuit current between the first and the second of mentioned phases for current relation of phases voltages on the first stage of commutation, on the second stage it is being turned off the half of the switch that is unidirected with the turned on half of the switch at the first stage of commutation that is being turned off, it is being turned on the second half of the switch that is being turned on at the third stage, and it is being turned off the second half of the switch that is being turned off [L. Mazet, H. Boulant, J-J. Huselstein, C. Glaize, xe2x80x9cSwitching control in three phase matrix converters by discrimination of command ordersxe2x80x9d, in Proc. PEMC""98, vol 2, Prague, Czech Republic, 1998, pp.2-64-2-67]. During time intervals where it exists the complete certainty of the relation of voltages for those phases with switches being switched the step-by-step algorithm described provides the correct current commutation.
A disadvantage of this and above mentioned methods is lack of conditions for realization of a safe step-by-step algorithm of switches commutation on intervals where it is impossible to define exactly the relation between input phase voltages, especially under distortions of curves of mains phase voltages with connected switches which take part in commutation.
It is a primary object of the present invention to provide a method of current commutation by bi-directional switches of matrix converters with at least three input phases, which allows to avoid short circuits current through the switches and overvoltages on circuit elements due to eliminating commutations between switches being connected to those mains phases for which it is impossible to determine exactly the voltages relation between them for a given moment of time.
It is another object of the present invention to provide a method of current commutation by bi-directional switches of a three phasexe2x80x94to three phase MC with a space vector modulation (SVM), which provides the withdrawal of commutations between switches being connected to those mains phases, for which it is impossible to determine exactly the relation of voltages between them for a given moment of time, by means of forming a zero vector by a simultaneous turning on of all the switches of MC which are connected to input phase with the extremal module of voltage for a given moment of time.
It is a further object of the present invention to create conditions for using said method of commutation under forming of the output voltage with a deep control of a shift angle between input phase voltage and input current of MC.
And, finally, it is a further object of the present invention to provide a safe current commutation with optimization of switching rate during every SVM cycle under using the most useful mode of control of MC which is of the maximal possible relation between input and output voltages and performs the MC power factor close to 1.
The primary and other objects of the present invention are attained by the method of commutation of current by bi-directional switches of matrix converters with at least three input phases, the switches consisting of two separately controlled halves with the opposite directions conductivity, which implies that under transferring current from the switch being turned on at both directions which connects load to one input phase, into other switch which connects load to another input phase, it is being turned on the half of the switch that does not make a way for short circuit current flowing between said first and second phases for current relation of input phase voltages on the first stage of commutation, on the second stage it is being turned off the half of the switch that is unidirected with the half of the first switch being turned on at the first stage of commutation, it is being turned on the second half of the second switch at the third stage, and it is being turned off the second half of the first switch under the need to turn off the first one and turn on the second of the switches which are connected to mains phases, for which it is impossible to estimate exactly voltage polarity between them for a moment of step-by-step switching beginning, it is being identified the mains phase with the extremal module of voltage at given moment of time and it is being done an intermediate current commutation from the first switch into the third one which connects load to the said phase with the extremal module of voltage, whereupon current is being transferred into another switch.
In a three phasexe2x80x94to three phase MC with a space vector modulation (SVM) which provides synthesis of space vectors of output voltages (for instance, the phase voltagexe2x88x92Uout) and input current of zero and non-zero stationary vectors during every SVM cycle with using in turn two line voltages of the mains for non-zero stationary vectors forming and with dividing of a mains voltage period into six intervals with boundaries being determined by moments of input phase voltages polarity changing, under a necessity of transferring of MC load current from the first switch being turned on at both directions which provides connecting load to the first input phase, into the second switch which provides connecting load to the second input phase, according to the order of the switches halves step-by-step switching which is being set by voltages current relation for these input phases, and under impossibility of the exact estimation of the said voltages relation, the object of the invention is attained by forming a zero vector by the simultaneous turning on all the MC switches being connected to the third input phase which is the phase with the extremal module of voltage within a mains current interval, and thus it is being formed an intermediate current commutation from the first switch into the third switch which provides the connection of load to the said phase with the extremal module of voltage, whereupon current is being transferred into the second switch.
If it is being in turn used two input line voltages for non-zero stationary vectors forming, one of them is shifted by 60 electrical degrees relative to the other and is of minimal module within a current interval of the mains voltage, then the object of the invention is attained by grouping the non-zero vectors according to the sign of alternating of unidirected vectors or of forming of the extremal module input line voltage vectors, and if the last vector of the previous group and the first vector of the next group are non-unidirected and at least one of them is formed of the minimal module input line voltage within the interval of mains voltage period, then a zero vector is being placed between these groups in accordance with time of forming.
In the case of non-zero vectors forming by in turn using two input line voltages which are the extremal module ones within a current interval of mains period, the object of the invention is attained by grouping the non-zero vectors according to the sign of forming of the same input line voltage and a zero vector is being placed between these groups in accordance with time of forming, and under conjugating of SVM cycles the last vector of the previous period and the first vector of the next period is being formed of the same input line voltage.
A comparative analysis with other technical decisions shows that the proposed method differs from the known methods by the fact, that the proposed method allows to exclude current commutation between MC switches being connected to those mains phases for which it is impossible to determine exactly the voltages polarity between them for a given moment of time, and due to that fact the commutations are being done correctly, without short circuit current through the switches and overvoltages on circuits elements, under using the proposed method for the MC with SVM containing a zero component within every cycle, all the switchings of switches being correct and functionally expedient.